(1) Field of the Invention
The present invention relates to the measurement of size, concentration and size distribution of particles in fluids, and more particularly to the measurement of size, concentration and size distribution of particles in fluids by using acoustic speckle.
(2) Description of the Related Art
The measurement of the size and concentration of small particles in fluids is important in many industrial and diagnostic applications. Control or monitoring of operations such as crystallization, filtration, combustion, preparation of reaction feed streams, degree of reaction and the like, often depend on the ability to measure the size and concentration of small particles that are components of gas or liquid process streams.
A number of methods of measuring size and concentration of small particles are known in the art. Surveys are provided, for example, in Principles, Methods and Application of Particle Size Analysis, Syvitski, J., (Ed.), Cambridge University Press, London, (1991) and by Allen, T., in Particle Size Measurement, Chapman and Hall, London(1990).
Various optical methods that use coherent lightxe2x80x94lasers, for examplexe2x80x94as well as incoherent light have been reported to be useful for particle measurement in transparent or translucent fluids and in situations where particle concentration is low to moderate. Optical methods are reported that apply almost all wavelengths of light including wavelengths that are smaller, about the same as, or larger than the size of the particles to be measured. See, e.g., Etzler, et al., Part. Part. Syst. Char., 12 (1995).
Other well known particle measurement techniques include phase doppler anemometry, microscopy with optical scanning and image analysis, sieving, and particle counting by optical and electrical counters. Although all of these methods are useful in certain applications, each has limitations that make it difficult or impossible to use in certain situations.
Particle measurement by the use of ultrasonic energy has been shown to have useful advantages in certain applicationsxe2x80x94for example, in streams that are optically opaque, or where particle density is high. The attenuation of sound as a function of frequency (Acoustic Attenuation Spectroscopy) has been used in the Ultrasizer(trademark) device of Malvern Instruments Ltd., Worchester, U.K. In this method, a transducer inputs an acoustic signal having a certain frequency range into the fluid sample and a second transducer receives the attenuated portion of the original signal. Mathematical modeling permits the calculation of particle size and size distribution from the measured attenuation spectra.
Ultrasonic measurement of particles is also described by Behrman et al., On-line ultrasonic particle monitoring of brewing operations, MBAA Tech. Quarterly, 24:72-76 (1987); Bouts et al., An evaluation of new asphaltene inhibitors: Laboratory study and field testing, JPT, 782-787 (1995); de Boer et al, Screening of crude oils for asphalt precipitation: Theory, practice and the selection of inhibitors, SPE Production and Facilities, 55-61 (1995); Dickinson et al., Ultrasonic investigation of the particle size dependence of crystallization in n-hexadecane in water emulsions, J of Colloid and Interface Science, 142(1):103-110 (1991); and Holmes et al., A wide bandwidth study of ultrasound velocity and attenuation in suspensions: Comparison of theory with experimental measurements, J. of Colloid and Interface Science, 156:261-268 (1993); among others. U.S. Pat. No. 4,412,451 to Uusitalo et al., U.S. Pat. No. 4,509,360 to Erwin et al., U.S. Pat. No. 5,569,844 to Sowerby, and U.S. Pat. No. 4,706,509 to Riedel, also cover aspects of particle measurement by using ultrasonics.
In U.S. patent application Ser. No. 08/947,821, now U.S. Pat. No. 5,969,257, several of the present inventors described a novel use of ultrasonics to measure the size and concentration of small particles in fluids. In that method, a focused acoustic signal was transmitted into a fluid. Acoustic energy that is scattered by particles in the focal region of the signal is sensed by the same or another transducer. The signal of scattered ultrasonic energy is transformed from a time format to a frequency format and particle size and size distribution can be calculated by analysis of the magnitude of the signal as a function of frequency. This method was reported to be useful for the real-time measurement of particles in opaque liquids, such as crude oil.
A characteristic that has long been recognized in both laser and acoustic spectrometry is the phenomenon of xe2x80x9cspecklexe2x80x9d. General discussions of ultrasonic speckle have been provided by Wagner et al., Statistics of speckle in ultrasonic B-scans, IEEE Transactions on Sonics and Ultrasonics, 30(3):156-160 (1983), and Abbott et al., Acoustic speckle: Theory and experimental analysis, Ultrasonic Imaging, 1:303-324 (1979).
The presence of acoustic speckle has been traditionally regarded as an annoyance in applications such as ultrasonic medical imaging and efforts to minimize ultrasonic speckle have been reported by Wells et al., Speckle in ultrasonic imaging, Ultrasonics, 225-229 (September 1981), and Bamber et al., Ultrasonics, 41-44 (January 1986), and speckle minimization has been the subject of U.S. Pat. No. 4,561,019 (RE35,148) to Lizzi et al., U.S. Pat. No. 4,771,470 to Geiser et al., and U.S. Pat. No. 5,090,412 to Shimazaki.
More recently, however, there have been reports of the potential of deriving useful information from speckle patterns. See, e.g., Hong, S-K, and J-B Han, Considerations on speckle pattern interferometry of ultrasonic speckles, Ultrasonics, 35:329-332 (1997), for a description of the potential for using ultrasonic speckle to produce information about deformations and flaws in the internal parts of a material that can transmit ultrasonic waves. The same group report how known deformations produce changes in ultrasonic speckle patterns in Hong, S-K and Y. G. Ohr, Ultrasonic speckle pattern correlation interferometry using a pulse-echo method, J. Phys. D: Appl. Phys., 31:1392-1396 (1998),
The use of ultrasonic speckle signals to derive information about particles in fluid systems has been described by Nakajima et al. (U.S. Pat. No. 4,944,189) who report an ultrasonic speckle velocity measurement apparatus and method. A reported advantage of the method is that it can measure fluid velocity even at low velocities and the measurement is reported to be unaffected by the number or concentration of particles in the fluid.
Despite significant improvements in the methods and instruments available to measure the size and concentration of small particles in fluids, there remain certain applications where such measurement is difficult or impossible. For example, in fluids that are optically opaquexe2x80x94such as crude oilxe2x80x94or where particle concentration is high, and where measurements are required on a rapid basisxe2x80x94even on a real-time basisxe2x80x94and without dilution of the fluid, conventional particle measurement systems are unsuitable. Accordingly, it would be useful to provide a method and an apparatus for measuring particle size, concentration and size distribution of small particles in fluids that would be suitable for use even in optically opaque fluids. It would also be useful to provide such a method and an apparatus that could provide such measurements on a rapid, or real-time basis. It would also be useful if such a method and apparatus could perform these measurements without diluting the fluid in which the particles are carried.
Briefly, therefore, the present invention is directed to a method for determining the size of particles in a fluid comprising the steps of obtaining an acoustic speckle signal of the particles in the fluid and deriving the size of the particles in the fluid from the acoustic speckle signal.
The present invention is also directed to a novel method for measuring the concentration of particles in a fluid comprising the steps of obtaining an acoustic speckle signal of the particles in the fluid and deriving the concentration of particles in the fluid from the acoustic speckle signal.
The present invention is also directed to a novel method for determining the size distribution of particles in a fluid comprising the steps of obtaining an acoustic speckle signal of the particles in the fluid and deriving the size distribution of the particles from the acoustic speckle signal.
The present invention is also directed to a novel method for determining the degree of particle agglomeration in a liquid containing particles that produce individual speckle events, the method comprising obtaining, for two or more individual speckle events, an individual acoustic speckle signal from the liquid and determining the degree of agglomeration from the acoustic speckle signal.
The present invention is also directed to a novel apparatus for determining the size distribution of particles in a fluid comprising a signal generator capable of generating an electrical signal; a transducer that can translate the electrical signal from the signal generator into an acoustic input signal that is introduced into the fluid; a sensor that can sense acoustic energy that is scattered by the particles in the fluid in response to their interaction with the acoustic input signal and provide an output electrical signal that is related to the sensed acoustic energy; an oscilloscope that is capable of acquiring the output signal in an amplitude versus time domain and transforming the signal to a magnitude versus frequency domain; and a computer that is programmed with an algorithm that is capable of identifying an acoustic speckle signal in the frequency domain and relating the magnitude and/or duration of the acoustic speckle signal to the size distribution of the particles in the fluid.
Among the several advantages found to be achieved by the present invention, therefore, may be noted the provision of a method and an apparatus for measuring particle size, concentration and size distribution that are suitable for use even in optically opaque fluids; the provision of such a method and an apparatus that could provide such measurements on a rapid, or real-time basis; and the provision of such a method and an apparatus that could perform such measurements without diluting the fluid in which the particles are carried.